1. Technical Field of the Invention
The invention relates to frequency transposition and is advantageously applicable, without limitation, in the radio frequency domain, for example in mobile telephony, in which radio frequency circuits frequently use frequency transposition devices, or frequency mixers, both for transmission and for reception.
2. Description of Related Art
For transmission, frequency mixers, that are actually frequency step up circuits, are designed to transpose base band information around the transmission carrier. For reception, frequency mixers are frequency step down assemblies.
Existing radio frequency receivers are based on a direct conversion architecture. In other words, with this type of architecture, the input signal is directly converted into base band without the need for any intermediate transposition. This type of architecture has indisputable advantages, particularly the lack of any external intermediate frequency filters.
However, there are several disadvantages that complicate the design of this type of receiver. One of these disadvantages is due to the fact that the mixer output signal band comprises low frequencies close to zero, in other words close to direct current. The result is then that the mixer has its own output voltage offset that forms an interference signal for the useful signal. Furthermore, amplification means with a global gain of a few tens of dB are generally interposed between the mixer and the analog to digital conversion stage. The result is that the voltage offset of the mixer can saturate the analog to digital conversion stage.
Furthermore, the radio frequency part of a mixer must have a high gain, good linearity and low standby current. In order to achieve these performances, means of polarizing this radio frequency stage are usually used such that the standby current varies with the input signal making the circuit more linear for a given standby current. Furthermore, in order to guarantee stable temperature performances, the means of polarizing the radio frequency stage of the mixer usually use currents proportional to the absolute temperature (PTAT currents). Thus, standby currents in the radio frequency stage of the mixer vary with the power of the input signal and with the temperature.
However, in particular, these variations in the standby currents at the radio frequency stage are the cause of a variation of the mixer output offset voltage, and consequently may make it more difficult to satisfy the specifications on this mixer output offset voltage.
There is accordingly a need for a solution to this problem. Such a solution should preferably avoid any variation to the mixer output offset voltage, particularly when standby currents of the radio frequency stage vary with the input signal and with the temperature.